The present invention relates to thermoplastic polymer blends capable of being molded into plastic articles with improved properties. The polyblends of the present invention contain a graft copolymer of a polymerizable polyolefin component. Such copolymers are hereinafter referred to as "polyolefin graft copolymers". In a preferred embodiment of the invention, the polymerizable monomer is a vinyl halide, such as vinyl chloride, and up to about 50 weight percent of another copolymerizable monomer. The preferred embodiments will be referred to as "vinyl halide-graft polyolefin polymers". The present blends also contain a polymer composition hereinafter referred to as "SMA polymers".
Polyvinyl halide, especially polyvinyl chloride, polymer are widely used thermoplastic materials having many favorable properties. Such conventional non-graft vinyl halide polymers do not have heat distortion temperatures which are sufficiently high to adapt such polymers to much more wide use. Moreover, such polymers, especially rigid polyvinyl halide polymers, do not have a high impact resistance at ambient or sub-ambient temperatures. Thus, at ambient temperature, i.e., at about 20.degree. C., corresponding to about 68.degree. F., the notched Izod impact resistance of vinyl haldide homo- and copolymers is only of the order of about 0.4 to less than about 1 ft-lb/in. At sub-ambient temperatures, e.g., down to -20.degree. F. or lower, the notched Izod impact resistance of these polymers becomes vanishingly small or negligible.
It has been previously proposed to add minor amounts of an appropriate polymer additive, or additives, to improve ambient impact resistance of conventional polyvinyl polymer compositions. Usually, such additives are useful in ranges from about 3 to about 15 percent by weight of the polyvinyl halide polymer. Among the materials which have been found acceptable as polyvinyl halide impact modifiers are SMA polymers. Such impact modifiers moderately enhance the ambient temperature impact resistance of conventional vinyl halide polymers, i.e., generally raise the ambient temperature notched Izod impact resistance of the polymer to about 2 to 10 ft-lbs./in. However, these impact modifier are relatively ineffective in imparting a satisfactory sub-ambient temperature impact resistance to the polymer, i.e., the -20.degree. F. notched Izod impact resistance of the polymer containing the impact aodifier is well below 1 ft-lb/in and usually is about 0.4 to 0.5 ft-lb/in.
Recently, vinyl halide-polyolefin graft copolymers have been developed to be a commercial reality. Such copolymer are produced by polymerization of vinyl halide (or a monomer mixture of vinyl halide and copolymerizable ethylenically unsaturated comonomers) in the presence of a polyolefin elastomer. Such reaction yields a polymer product which contains vinyl halide polymer chains bound, i.e., grafted at various sites along the chain of the trunk olefin polymer, as well as ungrafted vinyl halide polymer and ungrafted polyolefin. The graft polymer product, especially the graft polymer product prepared by a liquid phase bulk polymerization reaction, has improved impact resistance at both ambient temperature and sub-ambient temperatures compared to the aforementioned conventional, i.e., ungrafted, vinyl halide polymers even when the latter are blended with a conventional polyvinyl halide impact modifying polymer additive. The bulk polymerization-prepared graft polymer product is even distinguished from the corresponding graft polymer prepared by a non-bulk polymerization technique, e.g., suspension polymerization, by an enhanced impact resistance at both low and ambient temperature and by breakage by the desirable ductile breakage mode rather than by an undesirable brittle breakage mode.
Although such graft polymer possesses a sub-ambient low temperature impact resistance substantially greater than that of conventional impact modifier-containing vinyl halide polymer compositions, the improvement, especially at low temperature, in impact resistance is not sufficient enough to make such copolymers generally useful in a wide variety of applications at low temperature.
It has now been found that blends containing (1) a polyolefin graft polymer, prepared in a new way, especially vinyl halide polyolefin graft polymers, and more especially those produced in a mass polymerization reaction, and (2) SMA polymers have particularly useful properties.
The molar miscibility exhibited by the matrix phases of these polymeric components of the invention offers several advantages. The miscibility provides excellent mechanical compatibility. Superior weld line strengths and improved surface properties can be obtained when the polymeric components exhibit molecular miscibility. A problem of possible deterioration of the properties due to phase separation during or after processing may exist for an immiscible blend. This is likely in case of the injection molding process which typically uses very high shear rates. This problem is unlikely in a case where the polymeric components exhibit molecular miscibility.